Maternal worry about infant weight has inconsistently been reported as a breastfeeding barrier. Weight monitoring is a critical tool to assess adequacy of infant feeding. Yet, little is known about the intensity of maternal worry about infant weight or associated breastfeeding outcomes. To examine (1) the frequency and intensity of maternal worry about infant weight; (2) the relationship between worry about weight and use of artificial milk; and 3) the relationship between worry about weight and breastfeeding cessation. A prospective cross-sectional design was used. A questionnaire was completed by women in the United States (N = 287) from 12 web-based maternal support groups. Sixty-three percent of women (n = 182) had some worry about infant weight. Participants breastfeeding for the first time had more worry (p = .035). Participants still breastfeeding had less worry about weight compared to those who had stopped (67%, n = 147 vs. 41%, n = 28). Exclusive breastfeeding participants had less worry (p < .001) compared to those who supplemented with artificial milk. Increased worry was associated with the use of artificial milk within 1 week of birth (p < .001) and early breastfeeding cessation (p < .001). Worry about weight is a significant breastfeeding barrier. It is associated with first time breastfeeding, less exclusive breastfeeding, use of artificial milk, and earlier breastfeeding cessation. Lactating mothers need anticipatory guidance about expected neonatal weight changes and interventions to help relieve worry about infant weight.